What Hurts the Most was Being So Close!

So, for the past 4 or 5 days, I've been playing Gears 2, working on trying to get the Seriously 2.0 achievement (get 100k kills). The way I've been doing it is by repeating the opening section of the brumak section of the last act, which generally nets around 50 to 80ish kills per run.

In order to keep it from being too monotonous, I'll throw on a podcast to listen to and I'll usually go for about 4 to 5 hours a day before calling it quits (which is usually around 20k kills per day).

Last night, I had a bit of extra free time so I played a bit longer, ending up at 80k. I figured that by the time I got on earlier tonight (as of writing this post) I'll be able to get the last few kills to get the achievement and then I can move onto something else to work on in the game achievement wise.

But to my absolute dismay, I lost all my progress from last night (I had started at 60k before finishing at 80k)! I had assumed that each time I reloaded the last checkpoint, the game would automatically save without me needing to go back to the main menu, so by the time I was done for the night last night, I just turned off my Xbox. So I just lost 4 hours of work for nothing!!